The present invention relates generally to a method and system for enhancing digital video transmission to set-top boxes in the presence of burst noise and, more particularly, to a method for enhancing the performance of multichannel systems providing analog and digital programming to subscribers, and most particularly, to a method for enhancing the performance of multichannel AM-VSB (amplitute modulated vestigial sideband)/QAM (quadrature amplitude modulation) video transmission system.
Video signals sent to set-top boxes of cable TV networks are often subject to xe2x80x9cburst/impulse noisexe2x80x9d, originating from peak Composite-Second-Order (CSO)/Composite-Triple-Beat (CTB) distortions (CSO/CTB) and/or electrical interference, leading to both video xe2x80x9cblockingxe2x80x9d and visually degraded areas in the video picture. Recently, there have been many studies on the performance of 64/256-QAM channels (as known to those skilled in the art, xe2x80x9cquadrature amplitude modulationxe2x80x9d is a system which greatly increases the amount of information which can be carried within a given bandwidth; the technique is essentially a combination of phase and amplitude modulation, where, for example, 64-QAM has 8 amplitude states and 8 phase states) in the presence of nonlinear distortions in hybrid analog/digital (e.g., AM/QAM) video transmission systems. Most of these studies have analyzed the effect of clipping distortion on the bit-error-rate (BER) of a QAM channel using CW carriers from a multitone generator. However, a method of eliminating the negative effect of bursty CSO and CTB distortions, on the bit-error-rate (BER) of the QAM channel using modulated video carriers, has not been determined.
In fact, the problems associated with burst noise generated errors have been addressed only by using a convolutional interleaver as part of the error-correction scheme in a quadrature amplitude modulation (QAM) modem or receiver. In particular, a convolutional interleaver may be inserted between the channel encoder and the channel and is typically characterized by the number of shift registers, which is also called the xe2x80x9cdepthxe2x80x9d I (symbols), and by the symbol delay increment per register J. Due to memory cost and end-to-end delay for the transmitted symbols, in certain applications it is advantageous to limit the interleaver (I, J) values.
More specifically, robust transmission of 64/256-QAM channels over current hybrid fiber/coax cable TV networks is achieved with the use of a forward-error-correction (FEC) scheme in the QAM modem or receiver. For some digital video broadcast (DVB) applications, the FEC scheme consists of a FEC code, such as Reed-Solomon (R-S) T=8 (204,188) code, a convolutional interleaver, and a randomizer. Interleaving the R-S symbols before transmission, and deinterleaving after reception, evenly disperses the burst errors in time, thus enabling the burst errors to be corrected by the FEC in the QAM modem or receiver as if the errors were randomly distributed.
As described in greater detail in the article entitled xe2x80x9cRealization of Optimum Interleaversxe2x80x9d, by John L. Ramsey (Ramsey, J. L., xe2x80x9cxe2x80x9cRealization of Optimum Interleaversxe2x80x9d, IEEE Trans. Inf. Theory, IT16, 338-345 (1970)), an interleaver redistributes the channel symbols such that the symbols from a codeword are mutually separated by somewhat more than the length of a typical burst of errors, thus effectively making the channel appear to the decoder as a random-error channel. Ramsey further describes four realizations of interleavers that reorder a sequence of symbols such that no contiguous sequence of n2 symbols in the reordered sequence, contains any pair of symbols that were separated by fewer than n1 symbols in the original ordering.
Although the R-S code in the QAM modem cannot correct generated CSO/CTB burst errors without an interleaver, implementation using a larger interleaver in the modem or receiver, to address the problems associated with burst noise-generated errors, greatly increases the cost and thus, by itself, may not provide the optimal solution.
The present invention is therefore directed to the problem of reducing the effect of CSO/CTB distortions and burst/impulse noise on a transmitted digital signals, such as a QAM channel over a cable TV network.
In a preferred embodiment of the invention, a method to improve the performance of a hybrid analog and digital video transmission system selects a digital channel map based on the relative magnitude and frequency locations of nonlinear distortions and the analog channel frequency plan.
In a particular embodiment of the invention, the hybrid analog and digital is a multichannel AM-VSB/QAM video transmission system. And the digital channel map is selected based on the relative magnitude and frequency locations of CSO and CTB distortions and based on the analog channel frequency plan. In one particular embodiment, the relative magnitude and frequency locations of CSO and CTB distortions are determined based on a type of laser transmitter in the video lightwave transmission system, which can be either a Directly Modulated (DM) laser transmitter or an Externally Modulated (EM) laser transmitter.
In yet a further embodiment, the digital channel map selected downshifts each QAM channel center frequency to reduce the CSO and CTB distortions. In yet a further embodiment, the selected digital channel map is downloaded to the set-top box.
In another aspect of the invention, the determination of analog frequency plan indicates a harmonic related carrier (HRC) channel frequency plan or an interval related carrier (IRC) channel frequency plan. If the HRC plan is indicated, in accordance with one embodiment, the digital channel map selected downshifts each QAM channel center frequency by 3-MHz relative to the picture carrier frequency. If the IRC plan is implemented, again in accordance with one embodiment, when the CSO distortions are dominant, the digital channel map selected downshifts each QAM channel center frequency by 1.75-MHz relative to the picture carrier frequency; while when the CTB distortions are dominant, the digital channel map selected downshifts each QAM channel center frequency by 3-MHz relative to the picture carrier frequency.
In yet a further embodiment of the invention, a QAM modem in the video transmission system includes an interleaver, which may be variable, and the depth of the variable interleaver may be adjusted based on the CSO and CTB distortions.
In still a further embodiment of the invention, the headend of a hybrid analog and digital video transmission system, for enhancing digital video transmission, includes generating means, for generating a composite analog, digital and/or data signal, and transferring means, for transferring the generated signal. In the system, the transferring means determines the relative magnitude and frequency locations of nonlinear distortions, identifies the analog channel frequency plan, and selects a digital channel map based on the determination and identification.